Weak-lensing Detection at z ~ 1.3: Measurement of the Two Lynx Clusters with the Advanced Camera for Surveys

Abstract

We present a Hubble Space Telescope Advanced Camera for Surveys (ACS) weak-lensing study of RX J0849+4452 and RX J0848+4453, the two most distant (at z=1.26 and 1.27, respectively) clusters yet measured with weak lensing. The two clusters are separated by ~4' from each other and appear to form a supercluster in the Lynx field. Using our deep ACS i₇₇₅ and z₈₅₀ imaging, we detected weak-lensing signals around both clusters at ~4 σ levels. The mass distribution indicated by the reconstruction map is in good spatial agreement with the cluster galaxies. From the singular isothermal sphere (SIS) fitting, we determined that RX J0849+4452 and RX J0848+4453 have similar projected masses of (2.0+/-0.6)×10¹⁴ and (2.1+/-0.7)×10¹⁴ M_solar, respectively, within a 0.5 Mpc (~60") aperture radius. In order to compare the weak-lensing measurements with the X-ray results calibrated by the most recent low-energy quantum efficiency determination and time-dependent gain correction, we also reanalyzed the archival Chandra data and obtained T_X=3.8^+1.3_-0.7 and 1.7^+0.7_-0.4 keV for RX J0849+4452 and RX J0848+4453, respectively. Combined with the X-ray surface brightness profile measurement under the assumption of an isothermal β-model, the temperature of RX J0849+4452 predicts that the projected mass of the cluster within r=0.5 Mpc is 2.3^+0.8_-0.4×10¹⁴ M_solar, consistent with the weak-lensing analysis. On the other hand, for RX J0848+4453 we find that the mass derived from this X-ray analysis is much smaller (6.3^+2.6_-1.5×10¹³ M_solar) than the weak-lensing measurement.